add experimental async/await support. (#582)

This patch adds experimental async/await support to Tokio. It does this
by adding feature flags to existing libs only where necessary in order
to add nightly specific code (mostly `Unpin` implementations). It then
provides a new crate: `tokio-async-await` which is a shim layer on top
of `tokio`.

The `tokio-async-await` crate is expected to look exactly like `tokio`
does, but with async / await support. This strategy reduces the amount
of cfg guarding in the main libraries.

This patch also adds `tokio-channel`, which is copied from futures-rs
0.1 and adds the necessary `Unpin` implementations. In general, futures
0.1 is mostly unmaintained, so it will make sense for Tokio to take over
maintainership of key components regardless of async / await support.

tokio-async-await/examples/echo_client.rs

@@ -0,0 +1,53 @@
+#![feature(await_macro, async_await, futures_api)]
+
+#[macro_use]
+extern crate tokio;
+
+use tokio::net::TcpStream;
+use tokio::prelude::*;
+
+use std::io;
+use std::net::SocketAddr;
+
+const MESSAGES: &[&str] = &[
+    "hello",
+    "world",
+    "one two three",
+];
+
+async fn run_client(addr: &SocketAddr) -> io::Result<()> {
+    let mut stream = await!(TcpStream::connect(addr))?;
+
+    // Buffer to read into
+    let mut buf = [0; 128];
+
+    for msg in MESSAGES {
+        println!(" > write = {:?}", msg);
+
+        // Write the message to the server
+        await!(stream.write_all_async(msg.as_bytes()))?;
+
+        // Read the message back from the server
+        await!(stream.read_exact_async(&mut buf[..msg.len()]))?;
+
+        assert_eq!(&buf[..msg.len()], msg.as_bytes());
+    }
+
+    Ok(())
+}
+
+fn main() {
+    use std::env;
+
+    let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
+    let addr = addr.parse::<SocketAddr>().unwrap();
+
+    // Connect to the echo serveer
+
+    tokio::run_async(async move {
+        match await!(run_client(&addr)) {
+            Ok(_) => println!("done."),
+            Err(e) => eprintln!("echo client failed; error = {:?}", e),
+        }
+    });
+}